The invention relates to a process and an installation for preparing binder-free hot-briquets for smelting purposes from ferrous pyrophorous, finely divided solids, such as dry steelmaking filter dust or finely grained sponge iron from direct-reduction plants.
In making steel, for example by the oxygen lance process, the CO recovery stage collects in filters metallised dust which, depending on the metallic iron content, has pyrophoric or highly pyrophoric properties. Finely grained sponge iron, for example from direct-reduction plants, frequently also has highly pyrophoric properties. These properties make it impossible for said finely divided solids to be bunkered and processed in untreated form.
It is known to briquet customary filter dusts or finely grained sponge iron to which binders have been added. The materials already proposed for use as binders are such products as bitumen and other tar products, molasses, waste sulphite liquor and the like. The disadvantage of these binders is that they, by their presence, reduce the concentration of valuable constituents in the product and frequently introduce impurities, for example sulphur, unwelcome in a subsequent treatment process, or present environmental protection problems. Since the binders are required in large amounts, the costs, which are made up of the price for the binder itself, the transport and storage costs and a number of other costs, are considerable, so that the economics are put into doubt.
German Auslegeschrift No. 1,142,442 discloses a process for briquetting fine ores, in which the ore is mixed with a basic aggregate, such as magnesium hydrate, lime hydrate, limestone or dolomite, and the mixture, without a special binder having been added and in the absence of clay, is briquetted under a high pressure. According to the description part of this Auslegeschrift, the process can also be applied to filter dust (see column 3/4, Table 1). The description does not indicate whether or not this filter dust has pyrophoric properties. The disadvantages of this cold-briquetting process are the high briquetting pressures, compared with hot-briquetting, and the associated high performance required of the briquetting press, and uneven compaction, which results in an inadequate bulk density for the briquet, since problem-free re-use of the filter use in the form of briquets in converters or electrical furnaces requires a sufficiently high bulk density, which must be significantly above that of the metallurgical slag used.
German Auslegeschrift No. 1,123,351 describes a process for preparing briquets, in which ferrous compounds, such as blast furnace flue dust and other ferrous wastes and iron compounds, including natural iron ores, are partially reduced as oxygen is removed. The resulting product consists of iron oxides and metallic iron and is then briquetted by the application of heat and pressure alone, at a temperature below 850.degree. C. and under a pressure greater than 700 kg/cm.sup.2. The reducing agent used can be a reducing gas, such as CO or H.sub.2, such an organic material as starch or sugar, or such a ferrous compound as ore containing Fe and FeO partially reduced to metallic iron. This process has the disadvantage that before the finely divided ores or other finely divided ferrous materials can be agglomerated, as desired, it is necessary to carry out a reduction process, as in known direct-reduction processes. At some point during this reduction process, it is necessary to raise the finely divided materials to a temperature which permits a subsequent hot-briquetting stage. This procedure, however, cannot be applied to the pyrophorous, finely divided filter dusts or the finely grained sponge iron, since these are already in the metallised state, albeit cooled down to a temperature so low that hot-briquetting is no longer possible.
German Auslegeschrift No. 1,533,852 discloses a process for preparing sponge iron briquets, in which immediately after discharge from a rotary tube furnace the porous sponge iron is briquetted in the hot state at temperatures of 400.degree. to 800.degree. C. and under briquetting pressures of 1,500 to 3,000 kp/cm.sup.2 with the proviso that the paired briquetting temperature and pressure values are selected in such a way, within the specified ranges, as to produce a density of more than 5 g/cm.sup.3.
German Journal "Sprechsaal-Fachbericht", Volume 102 (1978), Issue 2, pages 58 to 64, also reports on the hot-briquetting of sponge iron. As in the preceding process, the starting material is sponge iron which, because of its hot-discharge temperature, need not be additionally heated before being subjected to hot-briquetting. Neither of these two processes is thus suitable for filter dusts or sponge iron whose discharge temperature is lower than the hot-briquetting temperature.
The object of the invention is then to compact ferrous pyrophorous, finely divided solids without binder in as energy-saving, efficient a way while allowing for the pyrophoric property of the materials and to make smelting possible.